1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an active-matrix type liquid crystal display device of a thin film transistor (TFT) type or the like which uses polysilicon.
2. Description of the Related Art
Among liquid crystal display devices, liquid crystal display devices of a thin film transistor (TFT) type have been popularly used as display devices for personal computers or the like. The liquid crystal display device includes a liquid crystal display panel and driving circuits which drive the liquid crystal display panel. The liquid crystal display panel is configured such that two substrates are arranged to face each other in an opposed manner, a gap is defined between these two substrates, and a liquid crystal composition is filled in the gap. The substrates which form the liquid crystal display panel include pixel electrodes and counter electrodes. When a voltage is applied between the pixel electrodes and the counter electrodes, the orientation direction of liquid crystal molecules present between the pixel electrodes and the counter electrodes is changed so that the optical transmissivity of the liquid crystal display panel is changed. The display is performed by making use of this change of optical transmissivity. The liquid crystal display devices of a thin film transistor (TFT) type includes a switching element for each pixel electrode and the voltage is supplied to the pixel electrode through the switching element.
With respect to the liquid crystal display device of a thin film transistor (TFT) type, there has been known a vertical field type liquid crystal display device in which the pixel electrodes are formed on one substrate and the counter electrodes are formed on another substrate, and a lateral field type (in-plane type) liquid crystal display device in which the pixel electrodes and the counter electrodes are formed on one substrate.
A voltage supplied to the pixel electrodes is supplied to the vicinity of the pixel electrodes through video signal lines and is connected to the switching elements. Further, signals which turn on or off the switching elements are supplied through scanning signal lines. In the liquid crystal display device of a thin film transistor (TFT) type, a plurality of video signal lines extend in the vertical direction and are arranged in parallel in the lateral direction. Further, a plurality of scanning signal lines are extend in the lateral direction by intersecting the video signal lines and are arranged in parallel in the vertical direction. The pixel electrode is formed in a region which is enclosed by two neighboring video signal lines and two scanning signal lines which cross the video signal lines. The pixel electrodes are arranged in a matrix array so as to form a display region. In the periphery of the display region, driving circuits which supply signals to the video signal lines and the scanning signal lines are formed.
As the switching element, a TFT which uses an amorphous silicon and a TFT which uses polysilicon (hereinafter referred to as “polysilicon TFT”) are known. With respect to the liquid crystal display device using the polysilicon TFTs, a liquid crystal display device which forms the driving circuits on the same substrate on which the pixel electrodes are formed (hereinafter referred to as “driving circuit integral type liquid crystal display device”) is known.
An image is inputted from the outside (for example, a personal computer) to the liquid crystal display device as video signals. The video signals contain data on the voltages (gray scale voltages) applied to respective pixel electrodes. In general, the video signals are either analogue signals or digital signals. In the driving circuit integral type liquid crystal display device using the polysilicon TFTs, the driving circuits of an analogue signal inputting type have been used conventionally. The driving circuit of an analogue signal inputting type receives the video signals in the form of analogue signals from the outside, sample-holds the analogue signals and, thereafter, outputs the video signals to the video signal lines.